Advanced composites are high strength, high modulus materials which are finding increasing use as structural components in aircraft, automotive, and sporting goods applications. Typically, they comprise structural fibers such as carbon fibers in the form of woven cloth or continuous filaments embedded in a thermosetting resin matrix.
Most advanced composites are fabricated from prepreg, a ready-to-mold sheet of reinforcement impregnated with uncured or partially cured resin. Resin systems containing an epoxide resin and aromatic amine hardener are often used in prepreg since they possess the balance of properties required for this composite fabrication process. State-of-the-art epoxy/carbon fiber composites have high compressive strengths, good fatigue characteristics, and low shrinkage during cure. However, most epoxy formulations absorb moisture which reduces their high temperature properties. As a result, they are not suitable for use at 300.degree. F. or greater in a moisture saturated condition. There is therefore a need for resin systems which afford composites which can retain a high level of properties at 300.degree. F. under such moisture saturated conditions.
Most prepreg resins designed for use at temperatures of 300.degree. F. or higher are made by combining bismaleimides of Formula I with liquid coreactants containing other reactive groups such as amines, epoxides, cyanates or comonomers containing --CH.dbd.CH.sub.2, &gt;C.dbd.CH.sub.2, or --CH.dbd.CH-- groups which can react or polymerize with the carbon-carbon double bonds of the maleimide groups. ##STR1##
In the most common bismaleimides, R is the residue of an aromatic diamine such as methylene dianiline or related derivatives.
A wide range of bismaleimide compositions based on differing aromatic residues or blends of bismaleimides can be used, however. All of these bismaleimides are solids or glasses at room temperature which require liquid coreactants in order to provide a processible and useful resin for composite fabrication.
The range of suitable liquid reactive diluents containing unsaturated carbon-carbon double bonds, however, is currently very limited. Many of the available diluents are restricted by their low boiling points and, therefore, high volatility; by their odor; by their toxicity and/or problems with skin irritation; by their poor ability to dissolve bismaleimides; by their high viscosity which, again, limits the bismaleimide solubility and also leads to little or no tack in the formulation; by their poor thermal stability or hydrolytic stability; by their incompatibility with other formulation modifiers; by their effect on the mechanical properties of the cured formulation; and/or by their tendency to cause rapid cross-linking and gellation of the bismaleimide formulation.
A need clearly exists, therefore, for new and effective reactive diluents which can overcome all or at least many of the above noted problems.